Dicarboxylic acids or dicarboxylic anhydrides having a norbornene or norbornane structure are useful as the raw materials for agricultural chemicals or as industrial raw materials. Dicarboxylic acids or dicarboxylic anhydrides having a norbornene or norbornane structure are known to exist as stereoisomers, which include an endo isomer and an exo isomer with different melting points and reactivity.

Dicarboxylic anhydrides having a norbornane structure such as 5-norbornene-2,3-dicarboxylic anhydride can be reacted with an amine compound and then subjected to a ring-opening polymerization to yield an imide compound with a high glass transition point and a low dielectric constant that is useful as an electronic material, but it is known that the polymer reactivity and the physical properties of the product polymer differ depending on the steric structure of 5-norbornene-2,3-dicarboxylic anhydride (for example, see Macromolecules, (U.S.), 25 (1992), p. 5150). Furthermore, it is also known that when a low dielectric constant polymer is obtained by an addition polymerization at the olefin portion of an imide derivative of 5-norbornene-2,3-dicarboxylic acid, the exo isomer exhibits superior reactivity to the endo isomer (for example, see Japanese Laid-Open Publication No. 2002-30115, Japanese Laid-Open Publication No. 2002-88120, and Japanese Laid-Open Publication No. 2002-284812). In addition, the tetracarboxylic acid that is a derivative of the exo isomer is an important raw material for soluble polyimide acid (for example, see Japanese Laid-Open Publication No. Sho 63-57589). In this manner, the differences in reactivity and the like between the endo and exo isomers of 5-norbornene-2,3-dicarboxylic acid or the anhydride thereof mean that their respective levels of usability vary depending on the application.
Generally, this 5-norbornene-2,3-dicarboxylic acid or the anhydride thereof is prepared by a Diels-Alder reaction between maleic acid or maleic anhydride and cyclopentadiene, which yields a product containing the endo isomer as the primary component with a small quantity of the exo isomer.
Accordingly, in order to obtain 5-norbornene-exo-2,3-dicarboxylic acid or the anhydride thereof, the mixture obtained from the above Diels-Alder reaction, comprising the primary component 5-norbornene-endo-2,3-dicarboxylic acid or the anhydride thereof together with a small quantity of 5-norbornene-exo-2,3-dicarboxylic acid or the anhydride thereof, is usually heated, either alone or in combination with decalin or the like, thereby converting the endo isomer to the exo isomer via a thermal isomerization (for example, see The Journal of the American Chemical Society, (U.S.), 1963, pp. 115 to 116). However, at a temperature of 140° C. to 150° C., the endo and exo isomers of 5-norbornene-2,3-dicarboxylic acid or the anhydride thereof reach an equilibrium state in which the ratio of endo isomer/exo isomer is approximately 54/46, meaning the pure exo isomer cannot be obtained solely by thermal isomerization. Consequently, pure 5-norbornene-exo-2,3-dicarboxylic acid or the anhydride thereof is generally obtained by repeated recrystallizations (for example, see The Journal of the American Chemical Society, (U.S.), 1967, 69, p. 6896). However in order to obtain high-purity 5-norbornene-exo-2,3-dicarboxylic acid or the anhydride thereof using this recrystallization method, either the recrystallization is repeated a number of times, or a large quantity of solvent is used during the recrystallization, and as a result, a problem arises in that the yield of the 5-norbornene-exo-2,3-dicarboxylic acid or the anhydride thereof is very low. Furthermore, extracting the less prevalent stereoisomer efficiently and with a high degree of purity from a mixture of the endo isomer and the exo isomer is difficult.
Furthermore, a method in which 5-norbornene-exo-2,3-dicarboxylic acid is obtained experimentally by a photoisomerization reaction at 300 nm, using ethyl alcohol as a solvent and triethylamine as a catalyst, has also been reported (for example, see Chemistry Letters (Japan), 1991, pp. 1173 to 1176). However, because this method uses an amine and an alcohol, a problem arises in that a reaction occurs between the alcohol and the acid anhydride.
In addition, a dicarboxylic acid having a norbornane structure or the anhydride thereof is generally prepared by hydrogenation of the 5-norbornene-2,3-dicarboxylic acid or anhydride thereof obtained using the Diels-Alder reaction described above, and the product contains the endo isomer as the primary component with a small quantity of the exo isomer. Accordingly, in a similar manner to that descried above, for a dicarboxylic acid having a norbornane structure or the anhydride thereof, extracting the less prevalent stereoisomer efficiently and with a high degree of purity from a mixture of the endo isomer and the exo isomer is difficult.